One of the problems with which manufacturers of copper foil (both of the electrolytic and rolled variety) are faced is the staining of the copper foil between the time of shipment and the time of use by the ultimate purchaser. This staining results from exposure of the copper to normal atmospheric and other conditions and, particularly for applications of the copper foil in critical areas such as printed circuitry, is an extremely undesirable characteristic.
In order to overcome this problem, it has been common in the past to subject copper foil to a stainproofing treatment, a typical such treatment involving the use of aqueous chromate solutions. While such treatments have been successful in overcoming the staining problems referred to above, however, the stainproofing treatment itself has, on occasion, created collateral problems.
One such problem has occurred in the etching of stainproofed copper foil for printed circuit applications. More specifically, it has been found that stainproofed foil, i.e., copper foil which has been subjected to a chromate stainproofing treatment, may render the foil resistant to an acid etching treatment such as the standard ferric chloride etch used to prepare printed circuit boards. This resistance to etching of the stainproofed copper foil, of course, renders it substantially unusable for printed circuit applications.